Reed Relays Can Be the Best Solution for Signal Switching in T&M
Signal switching is vital in test systems, and there are many options to accomplish the task. Here's why reed relay-based switching systems may be your best bet.
Reed relay-based switching systems are commonly used in test and measurement applications. Small size, high isolation resistance, hermetically-sealed contacts, and fast actuation time can be some of the main advantages of reed relays over other switches.
As a recent example, Pickering Electronics has released a miniaturized reed relay with a minimum stand-off voltage of 1,500 V. The device is housed in the mini-SIP package with a footprint of 12.5 mm by 3.7 mm and a height of 6.6 mm.
On Semiconductor chose Pickering's miniature HV reed relay to be at the heart of its IC test system. Image used courtesy of Pickering Electronics
The new reed relay can switch up to 0.7 A, 10 W. It has single-pole, single-throw (SPST) normally-open configuration with three coil voltage options: 3 V, 5 V, and 12 V. These relays are suitable for applications such as cable testers, mixed-signal/semiconductor testers, and high-voltage applications.
In this article, we’ll try to develop some insight into the importance of signal switching in test systems and review some of the advantages of reed relays over other options.
Why Is Switching Required?
Switching allows us to sequentially connect one instrument to the desired terminals of a device under test (DUT). An example is shown in the following figure.
Example of a 6 × 8 one-pole matrix. Image used courtesy of Keithley
In this case, a 6 x 8 matrix of switches (represented by small circles at the intersection of rows and columns) allows us to connect an oscilloscope, function generator, and power supply to any two terminals of the DUT. Rather than asking an operator to move the probes around the DUT, we can use a processor to control the switches and have a programmable test sequence. This is faster and can be less error-prone.
Moreover, switching can lower the system cost by sharing instruments between multiple channels of a test and measurement system. One might think that designing a switching function such as the one depicted above is a trivial job. However, we’ll see that this is not the case. In fact, you must understand the signals to be switched and the tests to be performed to have an appropriate switch system.
Poor Switching Can Affect Accuracy and Repeatability
A switching system that is optimized for a high-voltage/current application is not usually suitable for switching a high-frequency signal. For example, a test set-up that switches a high voltage to a capacitive load needs to incorporate a series resistance to restrict the charging current (i=Cloaddvdt). However, a low-voltage application requires attention to phenomena such as thermoelectric offset voltage created in the switch card and switch film contamination.
On the other hand, a switching system for high-frequency applications should maintain impedance matching to achieve accurate measurements. That’s why we cannot use the same test set-up to switch 500 V, 500 MHz at 10 A.
We’ll need to do the high voltage test and the high-frequency measurement on two different set-ups. Without considering the practical limitations of the switching system, one can easily end up with erroneous measurements.
There are several different switch types and configurations that can be used to implement the switching function of multi-channel measurement equipment. The main switching choices are electromechanical relays (EMR), reed relays, and solid-state relays (SSR).
In the rest of this article, we’ll review some of the advantages of reed relays over other switching options.
This is a measure of the rate at which a relay can reliably switch from one state to another. Compared to EMRs, reed relays have lighter and simpler moving parts and can switch faster by a factor of about 5 to 10.
The operating speed of a reed relay is in the range of 200 µs to 500 µs. This can be a substantial advantage over an EMR because it significantly reduces the test time in a production environment.
Note that in such applications, test time carries a significant dollar value. SSRs are much faster than reed relays and offer operating speed as low as 10s of microseconds.
Even in the open state, a very small current can flow through the switch. This can be modeled by a contact-to-contact insulation resistance as depicted below.
Depiction of the insulation resistance of a switch relay in the open state. Image used courtesy of Keithley
The insulation resistance of most relays is in the range of 1 MΩ to 1 GΩ. With many applications, the small leakage current that can flow through the insulation resistance should be negligible. However, when switching currents in the range of 1 µA or less, we might need to take these tiny leakage currents into account.
The following figure shows how the leakage current of channel 2 is subtracted from the source current (Is) that was supposed to be delivered to Load #1.
Diagram showing the channel 2 leakage current subtracted from the source current bound for Load #1. Image (modified) used courtesy of Keithley (PDF)
Unlike SSRs, reed relays and EMRs are based on mechanical contacts that are separated from each other in the open state. That’s why reed relays and EMRs act more like a perfect switch and offer a higher insulation resistance.
Frequency of Switched Signals
With SSRs, there is a trade-off between the switch parasitic capacitance and its on-state resistance. Larger devices with larger parasitic capacitance are required to reduce the on-state resistance. These large capacitors restrict the bandwidth and create unwanted signal coupling between nearby relays.
The capacitance of a reed relay is considerably smaller than that of an SSR and, hence, a reed relay can more easily handle higher frequency signals.
On-State Contact-to-Contact Resistance
Unlike most EMRs, reed relays have hermetically sealed contacts. Since contaminants cannot enter the critical contact areas, we can have more consistent switching characteristics.
This can be particularly important in low voltage switching applications (below about 100 mV). In these applications, the contamination can produce a film on the contact surface and increase the contact resistance.
The variation in the contact resistance can lead to erroneous measurements. Voltages greater than about 100 mV are sufficiently high to clear these contaminations. Note that the on-state resistance of an SSR can be non-linear and vary with the load current.
Reed relays can be the best solution for implementing the switching function of many test and measurement applications. They offer advantages such as small size, high isolation resistance, hermetically-sealed contacts, and fast actuation time.